Generally, handover (HO) means that a mobile station moves away from a radio interface of one base station into a radio interface of another base station. In the following description, a handover procedure in a general IEEE 802.16e system is explained.
In IEEE 802.16e network, in order to inform a mobile station (hereinafter abbreviated MS) of information (e.g., topology) on a basic network configuration, a serving base station (hereinafter abbreviated SBS) is able to broadcast neighbor base station information via a neighbor advertisement (MOB_NBR-ADV) message.
In the MOB_NBR-ADV message, contained are such system information on a serving base station and neighbor base stations as a preamble index, a frequency and a handover (HO)optimization availability, DCD/UCD (downlink channel descriptor/uplink channel descriptor) information and the like.
The DCD/UCD information contains informations that should be known to a mobile station in order for the mobile station to perform information communications in DL/UL (downlink/uplink). For instance, the informations may include handover (HO) trigger information, MAC (medium access control) version information of a base station, MIH (media independent handover) capability information of a base station and the like.
In a general MOB_NBR-ADV message, informations on neighbor base stations of IEEE 802.16e type are contained. Hence, informations of neighbor base stations of types except UEEE 802.16e may be broadcasted to mobile stations via SII-ADV (service identity information advertisement) message. Accordingly, the mobile station may be able to acquire informations of a heterogeneous network base station by making a request for a serving base station to send the SII-ADV message.
In the following description, a procedure for a mobile station, which has acquired information on a neighbor base station by the above-described method, to perform a handover on IEEE 802.16e network is explained in detail.
Generally, a handover procedure on a general IEEE 802.16e network may mainly consist of three kinds of procedures including handover (HO) initiation & preparation, handover (HO) execution and handover (HO) completion.
One example of a basic handover procedure, which may be configured in the above manner, is described with reference to FIG. 1 as follows.
FIG. 1 shows one example of a handover procedure executable in a general IEEE 802.16e system.
Referring to FIG. 1, first of all, a mobile station (MS) is connected with a serving base station (SBS) and is then able to perform a data exchange [S101].
The serving base station periodically broadcasts information on a neighbor base station neighboring to the serving base station to the mobile station via MOB_NBR-ADV message [S102].
In the course of communicating with the serving base station, the mobile station is able to start scanning candidate base stations (candidate HO BSs) using a handover trigger condition. If such a handover condition as a prescribed hysterisis margin is exceeded, the mobile station is able to make a request for a handover procedure execution to the serving base station by sending a handover request (MOB_MSHO-REQ) message [S103].
The serving base station is able to notify the handover request made by the mobile station to the candidate base stations (candidate HO BSs) contained in the MOB_MSHO-REQ message [S104].
Each of the candidate base stations (candidate HO BSs) takes a preemptive step for the mobile station having made the request for the handover and is then able to deliver informations related to the handover to the serving base station via HO-RSP message [S105].
The serving base station is able to deliver the handover related informations, which are obtained from the candidate base stations (candidate HO BSs) via the HO-RSP messages, to the mobile station via handover response (MOB_BSHO-RSP) message. In this case, in the MOB_BSHO-RSP message, such information for performing the handover as an action time for the handover, a handover identifier (HO-ID), a dedicated handover (HO) CDMA ranging code and the like can be included [S106].
Based on the information included in the MOB_BSHO-RSP message received from the serving base station, the mobile station is able to determine a target base station among the candidate base stations. Subsequently, the mobile station is able to attempt a ranging by transmitting CDMA code to the determined target base station [S107].
Having received the CDMA code, the target base station is able to transmit a success or failure of the ranging and physical correction values to the mobile station via a ranging response (RNG-RSP) message [S108].
Subsequently, the mobile station is able to send a ranging request (RNG-REQ) message for authentication to the target base station [S109].
Having received the ranging request message, the target base station is able to provide the mobile station with such system information usable by the corresponding base station as CID (connection identifier) and the like via a ranging response message [S110].
If the target base station successfully completes the authentication of the mobile station and sends all update information, it may be able to notify a success or failure of the handover to the serving base station of the mobile station via a handover complete message (HO-CMPT) [S111].
Thereafter, the mobile station is able to perform information exchange with the handover performed base station [S112].
The above-described handover process is assumed as performed between a mobile station and a base station in accordance with IEEE 902.16e specification (e.g., WirelessMAN-OFDMA R1 Reference System). For clarity of the following description, a system, to which a general technology including IEEE 802.16e specification is applied, is named ‘legacy system’. And, a mobile station, to which a legacy technology is applied, is named ‘YMS (yardstick MS)’ or a ‘legacy mobile station’. Moreover, a base station, to which a legacy technology is applied, is named ‘legacy base station’, ‘R1 BS’ or ‘YBS (yardstick BS)’.
And, a mobile station, to which a technology (e.g., IEEE 802.16m specification: WirelessMAN-OFDMA Advanced System) further advanced than a general technology is applied, is named ‘AMS (advanced MS)’ or ‘advanced mobile station’. And, a base station, to which the advanced technology is applied, is named ‘ABS (advanced BS)’ or ‘advanced base station’.
YBS just has a legacy zone (hereinafter abbreviated L zone or LZone) having a physical channel frame structure applied to a legacy system. And, ABS supports AMS only or may support both AMS and YMS simultaneously. In case that ABS support AMS only (e.g., WirelessMAC-OFDMA advanced system only), ABS just has an advanced mobile station support zone (hereinafter named M zone or MZone) having a physical channel frame structure applied to an advanced system. ABS supporting both AMS and YMS (WirelessMAC-OFDMA Reference System/WirelessMAC-OFDMA Advanced Co-existing System: legacy supportive) may operate in mixed mode. In the following description, this mixed mode operation is explained on the assumption that a legacy system and an advanced system include IEEE 802.16e system and IEEE 802.16m system, respectively. ABS operating in mixed mode has both a legacy zone and an advanced mobile station supportive zone. An uplink (UL) region in mixed mode may be able to operate in a manner of being divided into L zone and M zone by TDM (time division multiplexing) or FDM (frequency division multiplexing).
Moreover, AMS is able to receive services from ABS and YBS both. In particular, the AMS is able to receive a service via one of an advanced mobile station supportive zone and a legacy zone. And, the AMS is able to perform both a handover performing process defined in a legacy system and a handover performing process defined in an advanced system. In the following description, this mixed mode operation is explained on the assumption that a legacy system and an advanced system include IEEE 802.16e system and IEEE 802.16m system, respectively.
Generally, while AMS is receiving a service in MZone of a serving ABS capable of supporting AMD and YMS both, it may happen that the AMS has to perform a handover into a legacy base station or LZone of another ABS. In doing so, a process of a network reentry into a target base station (i.e., YBS or LZone of ABS operating in mixed mode) may follow a procedure defined in the legacy system. Before following the defined process, it may be necessary for the AMS to acquire information on a neighbor legacy base station (or a legacy system supportive base station).
However, if a serving base station broadcasts information on a neighbor YBS via a neighbor advertisement (AAI_NBR-ADV) message, duplicative information is broadcasted from the LZone via a neighbor advertisement (MOB-NBR-ADV) message. In particular, a list of neighbor YBSs is broadcasted from the LZone of the ABS for YMSs. If the list of the neighbor YBSs is broadcasted from MZone of the ABS as well, the base station broadcasts the duplicative information so that the mobile station receives the duplicative information.